The present invention relates to printheads for matrix impact printers employing printing blades as the printing mechanism.
The use of flat, planar printing blades arranged in a stack to form a printhead in a dot matrix impact printer is well known in the art, such a construction being illustrated in U.S. Pat. No. 4,129,390. The printing blade typically comprises a rim portion carrying a flat coil therein and connected by a pair of flexible mounting arms to a mounting portion. Projecting from one side of the coil rim is an elongated printing arm carrying a printing stylus at the distal end thereof. In operation, the stack of printing blades is disposed in a magnetic field extending substantially normal to the planes of the coils. When a current is selectively applied to one of the coils in a predetermined direction, the rim and the printing arm thereof move in a printing direction for impacting a record medium, such as paper, through an inked ribbon to perform the printing function in a well-known manner, this movement being accommodated by the flexible mounting arms.
It is known in such prior art printheads to provide small apertures in the tips of the printing arms to serve as a barrier to prevent wicking of ink between adjacent printing arms into the coil regions of the printing blades. In one such arrangement, disclosed in the aforementioned U.S. Pat. No. 4,129,390, the printing tips of adjacent printing blades in the stack have oppositely-curved portions which define a small aperture or separation area therebetween to perform the anti-wicking function.
However, it has been found under certain circumstances that these very small apertures in the printing tips of the printing blades are insufficient effectively to prevent undesirable wicking of ink along the printing arms. Furthermore, it has been found that where fabric ink ribbons are used, tiny particles of the ribbon fabric and other debris from the imprinted paper and the like tend to accumulate in the small apertures in the tips of the printing arms and clog them with a material which has a character of a saturated felt and serves to fill the separation and defeat any anti-wicking function it may have otherwise been capable of performing.